1. Field of the Invention
The invention concerns a satellite attitude control and stabilization system.
2. Description of the Prior Art
A satellite in any orbit is subject to slowly varying external disturbance torques related to its environment, the major causes of which are:
atmospheric drag (especially in low orbits), PA0 solar radiation pressure, PA0 the terrestrial gravity gradient, PA0 the terrestrial magnetic field.
These torques generally appear because the satellite is not symmetrical in terms of its mechanical design or geometry or the mechanical or optical properties of the materials used.
In the case of a spin-stabilized satellite the action of such disturbances is retarded by the gyroscopic stiffness resulting from the rotation of the satellite body.
In the case of a three-axis stabilized satellite this stiffness is usually created artificially by means of momentum wheels. The effect of external torques--variation of the kinetic moment vector--can be cancelled out by controlling the speed of the wheels in an appropriate way (to create a speed differential). When the limiting speed is reached the wheels are "desaturated" (differential speed) using attitude control actuators: usually thrusters, solar sails or magnetic coils.
The main disadvantage of using thrusters is that this causes sudden changes in attitude which take a long time to be damped out and interrupt the mission, which is especially irksome in the case of terrestrial observation.
Use of magnetic coils allows continuous compensation. However, the low value of the magnetic field (approximately 10.sup.-7 Tesla) and the magnitude of the torques to be compensated (10.sup.-5 to 10.sup.-4 Nm depending on the asymmetry of the satellite) yield typical performance requirements of 100 to 1,000 ampere-turns.m.sup.2 and therefore high magnetic coil masses (10 to 20 kg or even higher). The use of magnetic coils is therefore usually limited to control with respect to a single axis of the satellite, for desaturation of the momentum wheels.
Some satellite configurations have a high degree of asymmetry. For example, many terrestrial observation missions require cooling of the focal plane of the infra red instruments to achieve satisfactory performance at IR (infra red) wavelengths. The temperatures required (&lt;100.degree. K.) are usually achieved by means of passive radiators situated on the North or South face of the satellite not facing towards the sun (assuming a geostationary orbit).
In the case of three-axis stabilization, these faces are usually employed to mount the two wings of an orientable solar generator. Appendages like these are incompatible with correct operation of a passive radiator, which requires a totally unobstructed "field of view" of the coldness of space.
This results in the use, for example, of an asymmetric configuration with a single-wing solar generator mounted on the face opposite the radiator(s). Compensating the high solar radiation pressure torque which this arrangement causes generally requires the installation of a "solar sail" at the end of a long boom, on the face opposite the solar generator wing. The magnitude (approximately 10.sup.-4 Nm) and the orientation of this torque mean that magnetic coils are not well suited to this application.
Apart from its high mass, this solution has the following disadvantages:
reduced performance of the cooling system since the sail is necessarily in the latter's "field of view",
the deployment of the sail constitutes a single failure point, which is to be avoided for reasons of reliability,
the impossibility of modulating the torque created in opposition to the solar radiation pressure torque, although the latter is subject to seasonal variation (due to the inclination of the plane of the terrestrial orbit).
The invention is directed to alleviating the aforementioned disadvantages by proposing a satellite carrying a solar generator and equipped with an attitude compensation device which is light in weight, reliable, compact and economical in terms of energy use and which is adapted to attenuate effectively disturbing torques, in particular those due to solar radiation pressure.